Micro-World Gems & Gemology, Winter 2022, Vol. 58, No. 4

Quarterly Crystal: Etching on Laurentthomasite

John I. Koivula and Nathan Renfro

Download PDF
Figure 1. This deeply etched 11.24 mm laurentthomasite crystal from Madagascar displayed a rich greenish blue color. The orange color patches are epigenetic limonitic residue. Photo by Annie Haynes; courtesy of Arkfeld Minerals.
 
 
Figure 1. This deeply etched 11.24 mm laurentthomasite crystal from Madagascar displayed a rich greenish blue color. The orange color patches are epigenetic limonitic residue. Photo by Annie Haynes; courtesy of Arkfeld Minerals.

The micro-world of gems and minerals involves the study of not only fluid and solid inclusions but also any significant surface features caused by growth and/or etching. When a rough crystal is fashioned into a gemstone, most if not all surface features of any importance are removed during the lapidary process. So when we encounter an interesting gem crystal, we always take the opportunity to examine the natural surfaces for any interesting evidence of growth or dissolution.

With that in mind, we recently studied a rough-surfaced etched hexagonal crystal of laurentthomasite (figure 1) from Madagascar that measured 11.24 × 9.76 × 4.32 mm and weighed 3.71 ct. Laurentthomasite, with the chemical formula Mg2K(Be2Al)Si12O30, is a relatively recent mineral approved in 2019 by the International Mineralogical Association (IMA) as a new species in the milarite (osumilite) group. It was named after French mineral dealer Laurent Thomas.

The rough surface of the laurentthomasite prevented the application of standard gemological testing. With magnification, however, we were able to observe a uniaxial optic figure from the crystal in polarized light. As expected, the dichroism displayed was a greenish blue down the optic axis direction and greenish yellow perpendicular to the c-axis. Under UV radiation, the crystal’s reaction was inert.

Figure 2. Some of the dissolution features observed on the etched surface of the laurentthomasite crystal were reminiscent of decorative hexagonal tiles, as seen using differential interference contrast (left) and diffuse transmitted light (right). Photomicrographs by Nathan Renfro; field of view 1.44 mm.
 
 
Figure 2. Some of the dissolution features observed on the etched surface of the laurentthomasite crystal were reminiscent of decorative hexagonal tiles, as seen using differential interference contrast (left) and diffuse transmitted light (right). Photomicrographs by Nathan Renfro; field of view 1.44 mm.

Examination of the surface using differential interference contrast microscopy showed an abundance of hexagonal etch features, some with a rather dramatic architecture. Reflecting the hexagonal symmetry of the laurentthomasite, as shown in figure 2, these dissolution features were targeted for photomicrography.

John Koivula is analytical microscopist, and Nathan Renfro is senior manager of colored stone identification, at GIA in Carlsbad, California.